The impact hammer is a good idea. I would instead employ it on the cyl surface directly outside where the gland screws engage. TURN THE POWER DOWN and, holding the hammer tip located with gloved hand, use a short burst at 3 pts equally spaced. This will disturb the "nut"/"bolt" thread interface and help break the bond that has formed long term. - It will also stretch the metal slightly increasing the circumference of the nut/cyl. The gland screw may then be removable. If not, use different equi spaced impact points [and maybe more power] if you repeat. Short bursts only ... pull the trigger all the way in and let it go immediately - about 10 blows.
larry
Note that I addressed your cautionary points originally, altho I did not spell out reasoning as you have. The effects on side and end application are different [underlined]. The true ~ lurking danger of side impact is too many quickly repeated blows causing galling.the only problem with this is if the cyl barrel is thin (like most all kuboata cyls) and the OP gets a little rough......he'll likely distort and damage the threads and never remove the gland if this happens......if impact is needed on the outside of the barrel then I use just a regular hammer to the outside while using the spanner
Using the air-impact hammer parallel to the hyd rod (slightly tilted counter-clockwise) will give the same shock benefit without any distortion of the threads.......and the end of the tool can be slightly pointed to dig in without any harm